Re: [RFC PATCH] ALSA: core: Add DMA share buffer support

[Takashi Iwai]

On Tue, 22 Jan 2019 21:25:35 +0100,
Mark Brown wrote:
> 
> On Mon, Jan 21, 2019 at 03:15:43PM +0100, Jaroslav Kysela wrote:
> > Dne 21.1.2019 v 13:40 Mark Brown napsal(a):
> 
> > > It was the bit about adding more extended permission control that I was
> > > worried about there, not the initial O_APPEND bit.  Indeed the O_APPEND
> > > bit sounds like it might also work from the base buffer sharing point of
> > > view, I have to confess I'd not heard of that feature before (it didn't
> > > come up in the discussion when Eric raised this in Prague).
> 
> > With permissions, I meant to make possible to restrict the file
> > descriptor operations (ioctls) for the depending task (like access to
> > the DMA buffer, synchronize it for the non-coherent platforms and maybe
> > read/write the actual position, delay etc.). It should be relatively
> > easy to implement using the snd_pcm_file structure.
> 
> Right, that's what I understood you to mean.  If you want to have a
> policy saying "it's OK to export a PCM file descriptor if it's only got
> permissions X and Y" the security module is going to need to know about
> the mechanism for setting those permissions.  With dma_buf that's all a
> bit easier as there's less new stuff, though I've no real idea how much
> of a big deal that actually is.

There are many ways to implement such a thing, yeah.  If we'd need an
implementation that is done solely in the sound driver layer, I can
imagine to introduce either a new ioctl or an open flag (like O_EXCL)
to specify the restricted sharing.  That is, a kind of master / slave
model where only the master is allowed to manipulate the stream while
the slave can mmap, read/write and get status.


thanks,

Takashi